Mass spectrometry



Feb. 5, 1957 Filed Feb. 8,

R. V. LANGMUIR MASS SPECTROMETRY 2 Sheets-Sheet l /NPUT GA T/NG S/GNALH. v. 7 GA TED POWER SUPPL r AMPL/F/ER 7 PULSE PULSE 7 0 SENS/N6 FORMERFORMER MEA NS v. F. DELAY OSC/LLATOR L /NE /2 I LL? I:

IN V EN TOR.

ROBERT M. LANGMU/R BY yw /7%% A T TORNE V Feb. 5, 1957 R. v. LANGMUIR2,780,728

MASS SPECTROMETRY Filed Feb. 8, 1954 2 Sheets-Sheet 2 AMPLIFIER h. 1POWER SUPPLY PULSE 1 F FORMER OSCILLATOR F G. 5. F/ 6 6.

IN V EN TOR. ROBERT V. LANGMU/R ATTORNEY MASS SPECTROMETRY States PatentRobert V. Langmuir, Altadena, Califl, assignor, by mesne assignments, toConsolidated Electrodynamics Corporation, Pasadena, Calif., acorporation of California Application February 8, 1954, Serial No.408,879

3 Claims. (Cl. 250-413) This invention relates to mass spectrometry andmore particularly to an improved instrument of the type generallyreferred to as a time of flight mass spectrometer.

A mass spectrometer is an analytical instrument which functions toionize a sample to be analyzed, to spatially separate these ions inaccordance with their mass-tocharge ratio and to selectively sense therelative abunis the time of flight mass spectrometer. Instruments ofthis type as described in the art generally comprise an ion source, adrift tube and a collector electrode. Means are provided in the ionsource to project ions into and through the drift tube in mass pulseswhereby ions of differing mass segregate themselves in the drift tubeinto spatially separated ion bunches with each bunch com prising ions ofa given mass-to-charge ratio and dilfering from the mass of ions formingthe other bunches. These ion bunches strike and discharge at thecollector electrode.

One conventional way of resolving the ion bunches is to gate theamplification and sensing system connected to the collector electrode sothat it becomes sensitive only to pulses formed by the successivedischarge of ion bunches of a given specific mass. This can be done byinterconnecting the pulsing means by which the ions are projected fromthe ion source and the amplification and sensing system of the collectorelectrode to determine the proper time sequence of sensitivity of thecollector system. A second means of resolving the discharges of thecollector electrode is by feeding the amplified output of the collectorelectrode to a conventional cathode ray tube and adjusting the sweep ofthe tube so that only the output pulses developed by successivedischarge of ion bunches of a given specific mass are displayed on the 1By reason of the pres-' abundance of the ions in any one bunch. Thepresent invention is directed to improvements in a mass spectrometer ofthis type whereby this problem of induced currents is overcome. To thisend the invention contemplates in a mass spectrometer the combinationcomprising an evacuable envelope, an ion source, means associated withthe ion source for propelling ions from the .source in mass pulses, acollector electrode disposed in the envelope at a distance from thesource and in the path of ions propelled from the source, meansoperatively associated with the collector electrode to permit resolutionof ion masses discharged at the collector electrode,

and an electrostatic shield disposed closely adjacent the collectorelectrode.

The invention will be clearly understood with reference to the followingdetailed description taken in conjunction with the accompanying drawing,in which:

Fig. 1 is a diagrammatic illustration of a mass spectrometer inaccordance with the invention;

Fig. 2 is a diagram of a simple electrostatic circuit illustrating theproblem to which the invention is directed;

Fig. 3 is a graph of current flow with time in the electrostatic circuitof Fig. 2;

Fig. 4- shows the mass spectrometer of Fig. 1 provided with a cathoderay display means and without the gating circuit;

Fig. 5 shows a type of signal which may be obtained on a cathode raydisplay means without the electrostatic shield of the invention; and

Fig. 6 shows the type of signal produced on a cathode ray tube by theillustrated mass spectrometer provided with the electrostatic shield.

Referring first to Fig. 2, the circuit there shown comprises a batteryit) connected between electrostatic plates 11 and 12. A charged particle13 is shown in the region between the plates 11 and 12 and, as indicatedby the accompanying arrow, is considered to be in motion toward plate12. To move the particle 13 a distance x in time t the rate of work Wdone by the battery 10 is given by the expression W=iV 1 where W=workdone by battery;

V=the voltage between the plates 11 and 12; and i=the current flow inthe circuit.

The rate of work done on the particle is given by the expression v=thevelocity of the particle;

d=distance between the plates (in cm.);

x=a linear coordinate measured between plates 11 and t=the timecoordinate; and

V=the voltage between the plates.

Equations 1 and 2 above must be equal so that i=qv/d where v=dx/dt Thiscurrent i as given'by Equation 3 actually flows in the circuit while theparticle is crossing the gap. This current flow is a result of theinduced charges on the two plates, which change with time as theparticle moves across the intervening region. The shape of the currentpulse in this simple example can be shown to be as illustrated in Fig.3, i. e. substantially triangular because of the uniform increase invelocity as the particle traverses the gap. This subject is discussed indetail in Vacuum Tubes, K. R. Spangenberg, McGraw-Hill, 1948.

The foregoing description illustrates the problem encountered in a timeof flight mass spectrometer. Thus, a sharply defined bunch of chargedparticles will not give a sharp current pulse on arrival at thecollector electrode. This effect will be appreciated with reference tothe following description of a mass spectrometer in accordance with theinvention as illustrated diagrammatically in Fig. l. The description isbased upon the motion and detection of positive ions. Obvious variationsin potentials within the instrument will permit operation with negative'dotted" lines since the construction of theenvelope-and the usualevacuating and sample inlet'conduits are unimportant to an understandingof the present invention. An

ion source is disposed adjacent one, end of the envelope and includes aconventional electron gun for directing an electron beam 16 transverselyof the ion source region to anelectron target 17. .A number ofelectrodes 18, 19. and 20 aredisposed serially in the tube with the ofthe electron beam 16. A collector electrode 21 'is disposed adjacenttheopposite end of the tube. In accordance with the present invention anelectrostatic shield 22 is disposed immediately in front of thecollector electrode .21 and, as .illustrated,-is biased with a battery23.

The external circuitry of the instrument is shown diagrammatically ascomprising a variable frequency oscillator 24,.a pulse former 25 and ahigh voltage power supply. 26 connected serially, with the output of thehigh electrodes 18 and 19being. oriented on opposite sides,

voltage power supply being connected across a voltage divider 27. Theion source electrodes 18, 19 and Marc connected to the voltage divider27 so that a pulsed aecelerating potential is applied to theseelectrodes whereby ions are propelled from the source in mass pulses insynchronism with the applied pulse.

The variable frequency oscillator 24 is connected 7 through a delay line28 and a second pulse former 29 to deliver a gating signal to a gatedamplifier 31. Collector clectrode 21 is connected to the amplifier 31,the output of the amplifier being connected to' a conventional sensingsystem (not shown). The purpose of gating amplifier 31 by means of theoscillator 24, delay line 28 and pulse former 29 is to match theamplifier with the ion accelerating circuit so that the amplifier issensitive only during the intervals necessary to pick upsuccessivebunches of. ionsofagiven mass.

Ions propelled from the ion source in mass pulses segregate in the drifttube, this being the region between the source and the collectorelectrode into spatially separated bunches, each bunch consisting ofions of a given mass- 7 all of the other ion bunches. To resolve thegiven mass,

cnergization of amplifier 31 is matched through the'described circuitwiththe pulsed operation of the ion source so that the amplifier isrendered sensitive only in those intervals at which successive bunchesof the ion mass of interest strike the collector.

Delay line'28 causes theperiods of amplifier energization to lag behindthe pulses applied to the ion source by an interval suflicient to allowtravel of the ion bunch of thedesired mass to the collector.

Resolution of'ion masses in this type of an instrument is exceedinglydiflicult if theelectrostatic shield 22 is absent. The reason for thisdifiiculty is made apparent in the discussion 'of Figs. 2'and'3. Themovement of ions in theldrit't tube of the spectrometer induces acurrent in collector electrode which increases in magnitude as the ionsapproach this electrode. Because of the fact that; there are a number ofion bunches traversing the drift tube at a given time, this inducedcurrent is often unresolvablc. I have found. that collector electrode21-can be conveniently masked from the effects of such induced currentsby meansof the shield electrode 22 disposed immediately adjacentthereto. If the shield is kept at "approximately the same potential asthe collector electrode the current induced by the movement of chargedparticles in the drift tube is absorbed in effect by the mask, and theinduced current on the collector electrode will be extremely small untila particular bunch traverses the shield 22. Immediately upon traversalof the shield an ion pulse willinduce a largeand sharp current in thecollector as it traverses the narrow region between the shield-andtheicollector. For this reasonit is desirable that theshield be spacedfrom the collector a distance less than the spatial separation betweenadjacent ion bunches, 'A spacing of approximately .005 of ani nch issatisfactory for this purpose and is practical from a fabricationstandpoint.- W

The mass spectrometer of Fig. 1 is shown diagrammatically in Fig. 4connected to a cathode ray tube display system.- Like referencecharacters are used inFig. 4 to 'designateelements' common to Figs. 1and 4. In the apparatus of Fig, 4, collector electrode 21 is connected'directlyfthr'ough; an amplifier 32 to one set of deflecting platesofacathode ray. tube 40. The illustrated connectionshows the output ofamplifier 32 connected across the. vertical deflection plates of thecathode ray tube.

The horizontaldeflection plates are connected across f pulse former 29so that the horizontal sweep of the tube is matched with the pulsedacceleration from the :ion source. .A' sequence of ion bunches A, B, Cand D is shown in its approach to the collector electrode.

Without the-electrostatic shield 22, these ion bunches 'wiIlproduceatcollector electrode 21 a signal which, whendisplayed on acathode raytube, constitutes the aggregate ofthe several signals A, B, C and D inthe drawing. lFor purposes of illustration it is assumed that ionbunches A, B, Cand D are all present in thesame proportions,. whichcauses the respectively developed diselectrostatic shield 22. The actualcurrent signal on the tube.(curve '33) is the sum of the illustratedcomponent signals. p

Fig. .6 is another view of a cathode ray tube showing by-curve 34 thetype of signal obtained in the apparatus .ofIFig. 4 .in..the presence ofthe electrostatic shield 22.

By eliminatingove'rlapping induced currents, the shield permits displayof a resolved signal as displayed in Fig. 6

wherein the individual peaks produced by discharge of successive ionmasses on the collector electrode are observed separately.v

I claim:

1. In a mass spectrometer the combination comprising an evacuableenvelope, an ion source, a collector electrodedisposed in the envelopeand spaced from the ion source, means associated with the ion source forpropel: ling ions from Ithesource toward the collector electrode .inmasspulses, means operatively associated with'the collector electrodetosense ion masses discharged at the collector electrodeand anelectrostatic shield'disposed adjacent the face of thecollectorelectrodeat a distance ther efrornnotin excess of the approximate spacing ofadjacent ion masses inthe region of the shield. I

2. Ina mass spectrometer thc'combination comprising anevacuableenvelope, an ion source, a collector electrodedisposed in theenvelope and spaced from the ion source, means "associated with the ionsource for propelling. ions from the source toward the collectorelectrode in masswpulses, means operatively associated with thecollector electrodeto sense ion massesdischarged at'thc collectorelectrode, an electrostatic shield disposed adjacent the face of thecollector electrode and spaced therefrom such that at no time shall morethan one mass pulse have a capacity coeflicient to the collectorsubstantially greater than zero, and means for maintaining theelectrostatic shield at substantially the potential of the collectorelectrode.

3. In a mass spectrometer the combination comprising an evacuableenvelope, an ion source, a collector electrode disposed in the envelopeand spaced from the ion source, means associated with the ion source forpropelling ions from the source toward the collector electrode in masspulses, means operatively associated with the collector electrode tosense ion masses discharged at the collector electrode, an electrostaticshield disposed adjacent the face of the collector electrode and spacedtherefrom such that at no time shall more than one mass pulse have acapacity coeflicient to the collector substantially greater than zero,and means for maintaining the electrostatic shield at a potential whichis negative with respect to the potential of the collector.

References Cited in the file of this patent UNITED STATES PATENTS

